1. Field of the Invention
This invention relates to a process for purifying substituted phthalic anhydrides. More particularly, it relates to a process for purifying 4-methylphthalic anhydride derived from the dehydrogenation in the presence of bromine of 4-methyl-1,2,3,6-tetrahydrophthalic anhydride.
2. Related Art
Phthalic anhydrides are valuable raw materials for making various useful products. These anhydrides are useful as intermediates in the chemical synthesis of herbicides, and particularly in the synthesis of certain herbicides used to protect cereal crops. Other uses for these raw materials include polycyclic dyes, alkyd and epoxy resins, polyesters and plasticizers.
Various processes are known for preparing these substituted phthalic anhydrides. In many of these processes, a Diels-Alder addition product of a conjugated diene and maleic anhydride is dehydrogenated in the presence of bromine. The use of bromine in this dehydrogenation step introduces many impurities including hydrobromic acid and various brominated compounds. An effective economic means of purifying these substituted phthalic anhydrides, especially by reducing the hydrobromic acid and other brominated impurities, would be advantageous because the various useful products that are prepared from these anhydrides can be adversely affected by the presence of these impurities.
Chemical Abstract 84:105240Or, Purification of Phthalic Anhydride discloses the distillative purification of phthalic anhydride in the presence of zinc-containing materials as catalysts.
Chemical Abstract 70:77612d, Phthalic Anhydride Purification from Hydrobromic Acid discloses the purification of crude phthalic anhydride by heating at above 500.degree. F. with passage through a fractionating stripper column in countercurrent flow.